The printing of high quality full intensity text and line graphics has requirements that in are conflict with the printing of pictures. For text, high spatial resolution is required to eliminate jaggies in diagonal lines and to form fine features of a font. In contrast, the printing of pictures requires many levels of intensity or gray, but is less demanding in terms of spatial resolution.
Images can be defined in a number of encoding formats, optimized for the particular use of the image in a system. Each format has advantages and disadvantages. As an example, within a printing system, images may appear in either of continuous tone format or binary format, where each format has its own resolution. For the purposes of the present application, continuous tone format is an image defined by image signals or pixels, each pixel having more than two possible intensity or gray levels. Relatively coarse resolutions are possible, because the image can utilize the plural intensity level format to add information to the image. The format is considered highly portable. Continuous tone images can also utilize look up tables for tone reproduction correction (TRC).
In binary format much of the portability is lost as both intensity (via halftoning) and edge positioning must be described by full on or off pixels. In this format, via halftoning, intensity levels of gray are reproduced by combinations of binary spots in patterns which are larger or smaller depending on intensity level. Halftone formats require a trade off between the number of intensity or gray levels which can be used, and the frequency of the halftone pattern, which tends to become more visible as the number of intensity levels increases. Halftones are represented with binary signals and cannot be TRC corrected. Halftones patterns and their use are tied to specific machines, and are not generally considered portable. It is undesirable to represent text and line art in halftones, since edges tend to assume a ragged appearance. Edges can be represented with the binary format, but must be represented at a higher resolution than the halftone dot frequency to maintain high quality.
In rendering images for a halftone output device such as a laser printer, there is a tradeoff made in edge position control and representing continuous tone levels via halftoning. Binary formats are used in halftoning to represent intensity levels, but at the resolution of the continuous tone input pixel and halftone dot. Both are typically much lower resolution than the binary addressability of the output device. The image quality for some objects is reduced by the introduction of "jaggy", staircase like edges that have to be rendered at the lower continues tone pixel or halftone dot resolution. This invention describes a technique for concurrently encoding the edge information with the continuous tone pixel so that edges can be placed at the higher binary output resolution.
Each of the above described formats may have different resolutions. The resolution of the continuous tone pixels may be defined as CONRES. The resolution or frequency of halftone dots in a halftoned image is defined as DOTRES. Lastly, there is the resolution of the binary output device defined as BINRES. For a typical printing system: EQU BINRES&gt;CONRES&gt;DOTRES
FIG. 1 shows the output obtained printing a saturated (non-halftoned) object at CONRES, where the output edges are "jaggy". This would be further reduced in quality (i.e., it would have more jaggies) if the object were not saturated and halftoned. In the halftone case, due to the dot resolution (DOTRES), edges are placed at the halftone dot frequency that is generally lower than CONRES. FIG. 2 shows how the higher resolution of the output device (in this example 8.times. in the fast scan direction) that is normally used to render intensity levels via halftoning can be used to improve the edge placement. The visual impact of the jaggies are significantly is reduced.
It would be highly desirable to combine the edge enhancing features of high resolution BINRES with the high data content of CONRES, and to encode images in such a format. Additionally, due to the problems associated with image in DOTRES, it would be desirable to avoid using halftones until the last possible point in an image path.
U.S. patent application Ser. No. 08/353,763 to Harrington et al, teaches that edge control can be accomplished in continuous tone image objects, by treating the edges as high resolution bitmaps having the same shade value as the rest of the image. This is, however, a description of an image operation, and not a coding format for an image.
U.S. Pat. No. 5,307,180 to Williams et al., describes a system in which particular bits of data in a transferred data byte instruct the system about its operation.
Alternate methods, such as the proprietary CT/LW encoding from Scitex, require two raster planes. One plane contains areas where tone is important, the other edge important. This method also requires an operation to merge the two planes before printing.
References herein are incorporated by reference for their teachings